An uninterruptible power supply includes a converter for converting AC power from a commercial AC power source into DC power, an inverter for converting DC power into AC power, an AC filter for converting the waveform of an output voltage of the inverter into a sinusoidal wave, and first and second switches. One terminal of the first switch receives an AC voltage that has passed through the AC filter. One terminal of the second switch receives AC power from a bypass AC power source. The other terminal of the first switch and the other terminal of the second switch are both connected to a load.
During a bypass power supply mode, the first switch is OFF and the second switch is ON so that AC power from the bypass AC power source is supplied to the load through the second switch. During an inverter power supply mode, the first switch is ON and the second switch is OFF so that AC power generated by the inverter is supplied to the load through the AC filter and the first switch. At the time of change from the bypass power supply mode to the inverter power supply mode, the first and second switches are turned on so that AC power is supplied to the load from both the inverter and the bypass AC power source, and then the second switch is turned off (PTD 1: Japanese Patent Laying-Open No. 2012-120407).